1. Field of the Invention
The present invention relates to a CCD (charge-coupled device) color solid-state image pickup device.
2. Description of the Related Art
In relation to a CCD semiconductor solid-state image pickup element and a CMOS semiconductor solid-state image pickup element, color filters having different spectral transmission coefficients are stacked on a plurality of photodiodes arranged in a two-dimensional array, thereby enabling pickup of a color image.
In the color filters [red (R), green (G), and blue (B)] of primary color system, for instance, a B filter primarily permits passage of only light having a short wavelength of 470 nm or less. Hence, a photodiode of a light-receiving section with the B filter stacked thereon has sensitivity to B incident light. However, the B filter blocks light having other wavelength components (e.g., G and R), and hence G and R wavelength components that have entered the B filter are not subjected to photoelectric conversion. Thus, color filters of this type suffer a problem of a failure to effectively utilize all wavelength components.
In contrast, in the case where the color filters of so-called complementary color system are used, for instance, a yellow (Ye) filter permits arrival of light of G and R wavelengths at a photodiode of a corresponding light-receiving section; a cyan (Cy) filter permits arrival of light of B and G wavelengths at a photodiode of a corresponding light-receiving section; and a magenta (Mg) filter permits arrival of light of B and R wavelengths at a photodiode of a corresponding light-receiving section. Hence, as compared with a case where color filters of primary color system are used, effective utilization of incident light and enhancement of sensitivity can be achieved.
However, a signal output from each photodiode of the solid-state image pickup device using the color filters of complementary color system includes a mixture of a plurality of color signals, such as G and R, B and G, and B and R. Hence, an external signal processing circuit must perform processing for separating the signal into R, G, and B color signals. Accordingly, there is a problem that an image photographed and reproduced by a solid-state image pickup device using color filters of complementary color system is usually inferior in quality to that photographed and reproduced by a solid-state image pickup device using color filters of primary color system, in terms of color reproducibility, signal noise, or the like. Therefore, a digital still camera which photographs a still image frequently employs a solid-state image pickup device using color filters of primary color system.
In the solid-state image pickup device, color filters of respective colors are discretely arranged in a two-dimensional plane. Hence, there is a problem of a false color or moiré arising at a spatial frequency which is equal to or higher than a so-called Nyquist limitation. To alleviate this problem, there has hitherto been adopted a method for increasing the number of pixels in a unit image pickup area or a consecutive photoconductive film in lieu of discrete arrangement of light-receiving sections.
In principle, the configuration for discretely arranging color filters having different spectral characteristics as described in U.S. Pat. No. 3,971,065, which will be described below, encounters difficulty in solving color moiré or a false color. Problems, such as color moiré and false colors, cannot be solved until sensitivity to R, G, B visible light wavelengths is achieved at the position of a single pixel and separate identification of the respective R, G, and B color components can be achieved.
For this reason, there has already been proposed a method for identifying color signal components while utilizing optical characteristics of a silicon substrate in place of use of the color filters. Specifically, there has been proposed an identification method utilizing the following characteristics. Namely, the light absorption coefficient of a silicon substrate changes across a visible range from light of long wavelength (R) to light of short wavelength (B). Therefore, light of a wavelength range having a large light absorption coefficient is absorbed by a shallow area of the silicon substrate, and hence the light hardly reaches a deep area of the silicon substrate. Conversely, light of a wavelength range having a small light absorption coefficient reaches a deep region of the silicon substrate. Therefore, photoelectric conversion can be performed even at the deep area of the silicon substrate.
“A Planar Silicon Photosensor with an Optimal Spectral Response for Detecting Printed Material” by Paul A. Gary and John G. Linvill, IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. ED-15, No. 1, Jan. 1968. (hereinafter referred to as “Publication 1”) describes dependence of a photoelectric conversion characteristic of a photodiode on the depthwise position of a silicon substrate as well as on the wavelength of incident light.
An example of solid-state color imager comprised of three photo-sensitive layers, to which this idea has been applied is described in U.S. Pat. No. 4,438,455, which will be provided below.
The solid-state color imager with three photo-sensitive layers of U.S. Pat. No. 4,438,455 configured on the principle described in Publication 1 has a structure for extracting signals of three colors; i.e., R, G, and B. Without using color filters over-laid on the photo-sensitive elements, no light absorption of color filter material has arisen, and hence, incident light can be effectively converted into an electric signal.
As shown in FIG. 23 (corresponding to FIG. 3 of U.S. Pat. No. 4,438,455), U.S. Pat. No. 4,438,455 describes a structure 101 embodied by means of superimposing three photo-sensitive layers 102, 103, 104 and changing the depth of each photo-conductive layer against the incident light to apply the principle described in Publication 1 to the above structure.
The other example of CCD and MOS type solid-state color imager to which this idea has been applied is described in JP-A-1-134966, which will be provided below.
The solid-state color imager of JP-A-1-134966 configured on the principle described in Publication 1 has a structure of three story N+P photo-diode with different depth for extracting signals of three colors; i.e., R, G, and B, from one pixel. Without using color filters over-laid on the photo-diode elements, no light absorption of color filter material has arisen, and hence, incident light can be effectively converted into electric signal. Further, false signals or false colors, such as moiré, can be improved.
As shown in FIGS. 24A to 24C (corresponding to FIGS. 1(a) to 1(c) of JP-A-1-134966), JP-A-1-134966 describes a structure embodied by means of changing the depth of each N+P photo-diode to apply the principle described in Publication 1 to the above structure.
As shown in FIG. 24A, short wavelength light such as Blue is detected by the shallow N+P photo-diode 201. Long wavelength light such as Red is detected by the deep N+P photo-diode 203 as shown in FIG. 24C. The medium wavelength light such as Green is detected by the N+P photo-diode 202 locating in the depth of between the above two N+P photo-diodes as shown in FIG. 24B.
In the related-art interline CCD solid-state image pickup element, one reading gate section and one vertical transfer stage (one pixel is activated by three or four electrodes according to an all pixel reading method) are associated with one light-reading section. The area of the electric charge transfer path other than the light-receiving section is covered with a transfer-electrode. Therefore, no electrical contacts are provided in the light-receiving area, and no other peripheral circuits are provided for each pixel. Therefore, in the related-art CCD structure, signals corresponding to two or more different spectral sensitivities are read from one light-receiving section by utilization of a depthwise optical characteristic of the semiconductor substrate described in Publication 1. The signals cannot be subjected directly to charge transfer operation.